The present invention relates generally to semiconductors, and more particularly, to the scribing of semiconductor wafers using vision recognition.
Micro surface mount devices (micro SMDs) are semiconductor chips that are designed to be mounted directly onto a printed circuit board, Individual micro SMD chips are initially fabricated in wafer form using standard semiconductor processing techniques. Each micro SMD includes integrated circuitry and solder contact balls. Horizontal and vertical scribe lines of aluminum or some other type of conductive metal separate each micro SMD chip on the wafer.
A fabricated wafer undergoes a number of steps to prepare the micro SMD chips for mounting onto a printed circuit board. Initially the back surface of the wafer is grinded to reduce its thickness and then coated with an epoxy material. The epoxy prevents chipping of the wafer during dicing. It also provides a surface for marking the chips with part numbers, company logos, and the like. Next the wafer is probed and each micro SMD chip is individually tested to identify operational and non-operational devices. A dicing machine then saws the wafer, along the scribe lines, to separate the individual chips from one another. During the mounting process, the solder balls on a micro SMD chip are aligned with electrical contact pads on the board. The board is subsequently heated causing the metal of the solder balls to flow, forming an electromechanical joint between the chip and the contact pads on the board. In a final step, an underfill epoxy material is introduced between the chip and the board. The board is again heated, causing the epoxy to cure. The cured epoxy forms a seal around the chip that protects it from moisture and helps preserve the integrity of the joints. For more information on micro SMD packaging, see xe2x80x9cMicroSMDxe2x80x94A Wafer Level Chip Scale Packagexe2x80x9d, by N. Kelkar, Mathew H. Takiar and L. Nguyen, IEEE Transactions on Advanced Packaging, Special Issue on Wafer Level Packaging, pp. 227-232, Vol. 23, No. 2, May 2000.
In U.S. Pat. No. 6,245,595 entitled xe2x80x9cTechniques for Wafer Level Molding of Underfill Encapsulantxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein for all purposes, an improvement to the aforementioned micro SMB fabrication and mounting process is described. This improvement involves providing either a cured or a partially cured epoxy layer on the top surface of the wafer before it is diced. The epoxy layer not only protects the chips during handling, but also eliminates the need of the underfill epoxy material after the chip is mounted onto the printed circuit board. The epoxy layer formed on the top surface of the wafer includes a number of components, including an epoxy resin, a hardener, a catalyst, a filler material such as silicon particles and a dye. The silicon particles reduce the coefficient of thermal expansion of the epoxy to match that of the printed circuit board upon which the micro SMD will be mounted. When variations of temperature occur, the board and epoxy expand and contract at substantially the same rate. Without the filler particles, the rates of expansion and contraction would be different, resulting in potential failures of the joints over time. The filler particles and dye also tend to make the epoxy layer opaque which protects the micro SMD from exposure to light. Since many micro SMDs include analog circuitry that is light sensitive, the opaque epoxy layer helps maintain the proper operation of the chip in the presence of light.
The issue with the aforementioned improvement is that the opaque epoxy layer on the top surface of the wafer reduces the visibility of the scribe lines. It is therefore difficult for the dicing machine to recognize the scribe lines, often resulting in inaccurate cuts during dicing. Consequently damage to some of the chips during the cutting process may occur.
An apparatus and method for scribing wafers using vision recognition to enhance the recognition of the scribe lines through the opaque epoxy on the surface of the wafer is therefore needed.
To achieve the foregoing, and in accordance with the purpose of the present invention, an apparatus and method for scribing a semiconductor wafer coated with a substantially opaque material using vision recognition is disclosed. The apparatus includes a stage configured to hold a wafer, an imaging unit configured to generate an image of the wafer, and a computer configured to identify the coordinates of the scribe lines on the wafer from the image. During operation, the wafer is imaged using the imaging unit. The computer then identifies the coordinates of the scribe lines on the wafer from the image. Thereafter the coordinates are provided to a dicing machine which performs the dicing of the wafer. Accuracy is therefore improved since the dicing machine relies on the coordinates of the scribe lines as opposed to attempting to recognize the scribe lines through the opaque material. According to various embodiments of the invention, the imaging unit may use infrared, X-ray or ultrasound waves to generate the image of the wafer.